PBECIOUS STONES.	29
	of the ray of light between the two sheets of glass. Such an instrument is a simple form of the polariscope. More frequently two prisms of Iceland Spar cut and mounted in a particular way are used to polarise and analyse the light; such prisms are called "Nicol's prisms"; the prisms are mounted so as to be capable of complete rotation with regard to one another, and there are two positions in the complete revolution of maximum extinction of light; when the prisms are in one of these two positions, they are said to be "crossed." Now a singly-refracting body placed between crossed Nicol's prisms allows no light to pass whatever the position of the body, but in the examination of a gem we must use two or even three positions to make sure we are not dealing with a doubly refracting substance placed with the optic axis in the axis of the instrument. In examining a gem it must be first placed resting in the carrier on its small back facet, so as to give the light every opportunity of being transmitted through the stone. If there is a possibility that total internal reflection is preventing the light reaching the eye, we may overcome the possible fallacy by immersing the gem in a liquid of similar refractive index; for this purpose methylene iodide or mono-bromo-naphthalene may be used. When a doubly-refracting stone is placed in the right position, we shall find on rotating it that there are four positions in the 360° in which there is maximum lightness, due to the light being so altered in its passage through the gem that it can pass through the analyser. One other possible fallacy remains to be mentioned. Some singly-refracting substances show what is known as anomalous double refraction ; this is due to strains set up